The links to firmware.md disappeared earlier so this change
incorporates some of the the text there in under the Firmware heading
in software.md. Also changes the names of the commands in the state
machine to the same names as in the rest of software.md.
Mention the Zmmul extension and what it means and possible workarounds
for people developing with clang < version 15.
libhidapi-dev is required to build icestorm:
```
pi_pico_interface.c:5:10: fatal error: hidapi/hidapi.h: No such file
or directory
```
Tested with podman.
Signed-off-by: Marcello Sylvester Bauer <sylv@sylv.io>
